check if 2 sorted lists of string have any overlaping item in O(logn)

overlap.cpp

// precondition: a & b are both sorted
bool overlap(vector<string>& a, vector<string>& b) {
    auto i = a.begin();
    auto j = b.begin();
    auto n = upper_bound(i, a.end(), *b.rbegin());
    auto m = upper_bound(j, b.end(), *a.rbegin());
    while(i < n && j < m) {
        if(*i == *j) {
            return true;
        }
        auto nextj = lower_bound(j, m, *i);
        auto nexti = lower_bound(i, n, *j);
        i = nexti;
        j = nextj;
    }
    return false;
}